Benita Ferrero-Waldner said after talks with President Nursultan Nazarbaev in Astana that granting Kazakhstan the organization's rotating chairmanship "cannot be done at the expense of the OSCE's core values."
Ferrero-Waldner later said the EU hopes Kazakhstan will soon reform its media and election laws, and its constitution.
The OSCE is due to vote on Kazakhstan's chairmanship bid in December.
Nazarbaev sees the taking over of the OSCE's leadership as a way to boost the profile of Kazakhstan, which critics say is making little progress toward democracy.
The Kazakh presidential office said Ferrero-Waldner had invited Nazarbaev to visit Brussels in December.
Whether the visit will take place before the OSCE decides on Astana's chairmanship bid is unclear.